U.S. Pat. Nos. 5,258,556 and 5,169,994 describe the isolation of the product from the hydrogenation step (2,2,4,4-tetramethylcyclobutanediol) by either a distillation or crystallization technique. In the examples in these patents, the isolation method by crystallization is described as cooling the filtered, post-hydrogenation mixture to room temperature and separating the product from the solvent.
However, this crystallization method fails to produce a solid product from the post-hydrogenation mixture that can meet a number of stringent criteria necessary for the product to perform satisfactorily in downstream processes. These criteria generally relate to the yield of product, the shape of the crystal size distribution, the ease of isolation of the formed crystallization solids, the amount of adhering moisture remaining on the crystal surfaces after filtration, and the moisture content of the filtered product.
In general, the yield of product from a single pass crystallization operation should be maximized. A low yield of dissolved material is both materially and economically inefficient.
Ideally, the shape of the crystal size distribution generated in the crystallization zone should be such that the quantity of “fines” is minimized. “Fines” are particles that tend to plug the pores of the filter or that fill the gaps between larger particles on a filter, which results in reduced flow rate of liquid through the filter. “Fines” particles are known to require long filtration times and to produce dust when dried that provides a safety hazard.
Isolation of the formed crystallization solids is usually performed using a mechanical filtration device, such as a filter or centrifuge. The shape of the crystal size distribution should also be such that the driving force for filtration is minimized and the surface area required for the filtration is also minimized. This would yield a smaller filtration device that can operate at faster filtration rates.
Typically, the amount of adhering moisture remaining on the crystal surfaces after filtration should also be minimized. High surface moisture content increases the probability of impurity entrainment to successive operations.
In general, the moisture content of the filtered product should be low enough so that the successive drying operation has minimum energy demands and requires a short processing time.
U.S. Pat. Nos. 5,258,556 and 5,169,994 describe a batch process to crystallize 2,2,4,4-tetramethylcyclobutanediol from solution. However, for a large scale process, it is more economical to use a continuous process to isolate the product from solution as this eliminates the need for complex heat transfer equipment, seeding protocols and additional storage equipment typically used for batch operations. Continuous processes are also easier to control due to a small dynamic range of operation as compared to batch processing.
U.S. Pat. No. 7,989,667 discloses continuous crystallization of cis- and trans-2,2,4,4-tetramethyl-1,3-cyclobutanediol (TMCD) from isobutyl isobutyrate.
In the production of 2,2,4,4-tetramethyl-1,3-cyclobutanediol (TMCD)(CAS No. 3010-96-6), various co-products, solvents, and unconverted reactants accompany the TMCD product. The removal of these impurities from the TMCD product is advantageous for the production of TMCD as well as providing the capability to produce a high purity TMCD product. It is also desirable to have a choice of different solvents to select from in order to provide options for the isolation and purification of TMCD via crystallization. Proper solvent selection for the production of TMCD is important in removing the impurities and providing the capability to generate and recover a high purity TMCD product.
There is a need for processes that provide for the removal of the impurities and recovery of TMCD solids from a variety of solvents and solvent classes. Accordingly, the present invention is directed to addressing one or more of the needs described above.